Chromium is an unavoidable trace element of the raw material used in the manufacture of cement clinker, which is ground to produce cement. In particular, hexavalent chromium (“Cr VI”) may form in the oxidizing and alkaline burning condition of the cement kiln. Cr VI compounds are classified as extremely toxic because of their high oxidation potential and their ability to penetrate human tissue, thereby causing dermal sensitization, allergic reactions, and eczema. As Cr VI compounds have high solubility and are released when cement and water are mixed together, they tend to come into contact with human skin during the handling of wet cement and mortar.
It is desirable to reduce hexavalent chromium (Cr VI) to trivalent chromium (Cr III). This is because Cr III tends to precipitate from solution as a stable complex, thereby posing smaller risks as a serious dermal irritant. Indeed, a number of reducing agents are known. However, they tend to be effective at low pH levels rather than in the high pH environments of cementitious compositions.
Iron (II) sulfate has customarily been used for reducing chromium in cementitious systems. Iron (II) is oxidized to iron (III) in the process of reducing chromium. The actual dosage of Iron (II) sulfate needed for reducing chromium, however, is 10 times the stoichiometric amount required. This is due, in part, to the fact that iron (II) sulfate is added usually in dry form at a dosage of 0.3% to 0.5% (wt) of cement, and may be oxidized by moisture in the air. Moreover, such high addition of iron (II) sulfate in dry form is problematic. For one thing, dry materials are difficult to add accurately in cement milling operations. They also present a dusting problem that is logistically difficult to control, and the health hazards posed by the dust are cause for concern.
Equally significant, the excessive levels of iron sulfate needed are disadvantageous for cementitious systems because they increase water demand for workability, and extend setting time.
Typically, dry iron (II) sulfate cannot be added to cement at a temperature over 80° C., without risking considerable oxidation. As cement exits the mill at temperatures up to 130° C. and sent to storage at temperatures in excess of 80° C., iron (II) sulfate must be added in the later stages of the cement manufacturing and distribution process. Most often, iron (II) sulfate must be added just before the cement is packaged for shipment. This necessitates a further quality control step to check for chromium levels in the cement, and is inconvenient and costly for the cement manufacturer.
As an alternative to iron (II) sulfate, stannous (tin) sulfate can be employed as a chromium reducer. The required dosages of solid stannous sulfate are much lower than that of iron II sulfate (˜0.02% wt of cement). It is believed to be somewhat easier to use and to be more heat resistant and storage stable when combined with cement in the milling process. Still, however, there are similar logistical concerns with adding stannous sulfate as a dry powder into the cement milling operation.
Although stannous sulfate is water soluble, it quickly loses dosage efficiency when added as an aqueous solution to cement. The actual amount of stannous sulfate needed is at least double the amount that is required when stannous sulfate is added as a powder. Such a disparity precludes the use of stannous sulfate in solution form as a matter of economics.
Accordingly, in view of the foregoing disadvantages, the present inventors believe that a novel chromium reducing agent is needed that would require relatively low dosage addition rates and could be delivered and dispensed in liquid form.